Polymer materials are frequently used for preparing pipes suitable for various purposes, such as fluid transport, i.e. transport of liquid or gas, e.g. water or natural gas, during which the fluid can be pressurized. Moreover, the transported fluid may have varying temperatures. These pipes are usually prepared from polyolefins, such as medium density polyethylene and high density polyethylene.
Polyethylene (PE) pipes are generally manufactured by extrusion, or by injection moulding. The properties of such conventional PE pipes produced by extrusion or injection moulding are sufficient for many purposes, although enhanced properties may be desired, for instance in applications requiring high pressure resistance, i.e. pipes that are subjected to an internal fluid pressure for a long and/or short period of time.
According to ISO 9080, PE pipes are classified by their minimum required strength, i.e. their capability to withstand different hydrostatic (hoop) stress during 50 years at 20° C. without fracturing. The standard provides a definitive procedure incorporating an extrapolation using test data at different temperatures analyzed by multiple linear regression analysis. The results permit the determination of material-specific design values in accordance with the procedures described in the relevant system standards. This multiple linear regression analysis is based on the rate processes most accurately described by log10(stress) versus log10(time) models. Thereby, pipes withstanding a hoop stress of 8.0 MPa (minimum required strength MRS8.0) are classified as PE80 pipes, and pipes withstanding a hoop stress of 10.0 MPa (MRS10.0) are classified as PE100 pipes.
Advanced pipe materials should nowadays conform to pressure resistance levels higher than PE80. One attempt to meet these requirements has been the increase of the density of the polyolefin composition used for such pressure pipes. However, by increasing the density, slow crack growth resistance (SCGR) is reduced which will result in earlier brittle failure and thus has a negative impact on the minimum required strength (MRS) rating as well. The density increase also induces a flexibility decrease.
Further properties which are desirable to improve or to maintain at a high level are processability, tensile modulus, short term pressure resistance, and impact properties of the pipe material.
It is today difficult to produce a flexible pipe which can withstand high pressure for long times while at the same time satisfying criteria of processing, slow crack growth resistance, and cold temperature impact strength of PE100 class materials.
Therefore, it is an object underlying the present invention to provide pipes comprising polyethylene resins which can withstand high pressures for long times while at the same time satisfying criteria of processing, slow crack growth resistance, and cold temperature impact strength of PE100 class materials.